The present invention relates to the detection of particles and asperities on a workpiece surface. Specifically, the invention combines a heterodyne interferometer with darkfield illumination or Lloyd's mirror detection for detecting particles or asperities on a surface.
Darkfield inspection tools are the current state-of-the-art particle detectors for surface contamination. These tools rely upon the technique of establishing a distinctive interference node at the workpiece surface using grazing angle illumination. As a result, only particles or features having a profile higher than the dominant mirror plane of the surface are illuminated and scatter light. Certain of the state-of-the-art particle detectors detect the scattered light at grazing angles by use of a process called a Lloyd's mirror. Even the latter technique causes particles or features to be preferentially detected the greater the profile from the workpiece surface.
All of these tools perform under the assumption that the detectors will not measure any light when there are no particles on the surface. Efforts are made to baffle out room lights and to absorb stray scatter to assure that no light is measured in the absence of particles on the surface. For many types of products being inspected, such as rough metal film surfaces, the scattered light from the film being inspected that reaches blackened surfaces within the inspection tool and is reflected to be scattered into the detectors is the dominant source of background light and is the primary limitation affecting detector sensitivity.
A problem arises when these darkfield techniques are attempted to be used in in-situ measurements. Namely, light must travel through windows in a processing chamber. Moreover, typical processing chambers are light reflecting and not light absorbing. Additionally, the process being performed in the chamber often causes light emission. The combined effect is that there are too many scattered light sources for in-situ particle inspection using darkfield measurement techniques.
In addition to in-situ measurement inside manufacturing process tools, it is sometimes desirable to detect particles or asperities on a surface in combination with other inspection processes. For example, an optical microscope review station fitted with a surface particle detector would enable an operator to rapidly assess both the existence and type of contamination present on the surface. The addition of a particle detector to a scanning electron microscope or focussed ion beam tool would allow both the morphology and composition measurement of particles to be made without requiring an initial detection of the particles and the subsequent realignment of the workpiece with sufficient accuracy to enable re-detection of the particles in the scanning electron microscope or focussed ion beam tool.
Doubly darkfield microscopy is described in an article entitled "Doubly Darkfield Microscopy" in IBM Technical Disclosure Bulletin, volume 30, no. 6, November 1987 at page 334. Darkfield scanning is disclosed in U.S. Pat. No. 4,610,541 entitled "Foreign Substance Inspecting Apparatus."
An apparatus useful for particle detection having the scattered beam and the reference beam at the same wavelength is described in U.S. Pat. No. 5,030,842 entitled "Fine-Particle Measuring Apparatus." The patent describes two detection systems. In one system both the reference beam and the probe beam are reflected from the surface and are combined at a detector. This brightfield-brightfield system is ineffective for particle detection because the signal produced at the photodetector will reproduce the original laser modulation, irrespective of whether a particle is present or not. In the other system the photodetector receives only darkfield scatter from the workpiece surface. While this configuration is operable, it does not produce stray light rejection, because there is no optical beam coherent with the darkfield scatter incident on the detector to produce either homodyne or heterodyne amplification of the scattered signal.
In another arrangement both the probe beam and the reference beam have the same wavelength, producing homodyne instead of heterodyne amplification. In this arrangement the probe beam is modulated in intensity. An embodiment comprises, for example, a Bragg cell which transmits light to the surface only when the RF drive signal to the cell is turned off, so that amplitude modulation of the RF signal to the Bragg cell, in turn, modulates the intensity of the probe beam. While operable this embodiment is not optimal. Interferometric amplification works only when the reference signal frequency and the signal to be amplified have the correct phase relationship. In heterodyne amplification, all possible phase relationships are detected because the reference signal and the signal to be amplified have different frequencies. In homodyne amplification, since the particle scatter signal can have random phase, the reference and the signal to be amplified will not interfere properly a substantial fraction of the time.
The present invention overcomes the described limitations by combining the stray light rejection capability of heterodyne techniques with the asperity detection capability of darkfield illumination with or without Lloyd's mirror detection.